Hardness reference blocks are used for annual verification and calibration of hardness testing machines, the periodic (daily) check (see page 7) and sometimes for overtaking of hardness scales on a hardness testing machine. That's why they are a necessary help of industrial Quality Management. Only the use of high quality, precise hardness reference blocks calibrated to applicable standards can ensure the functionality and relative reliability and accuracy of measurement of a hardness testing machine.
The quality requirements for hardness reference blocks in respect of the necessary homogeneity, microstructure stability, uniformity of hardness, surface quality, are described in the corresponding international, european and national standards, and are easily exceeded by the Buderus blocks. The limits for the single parameters were much reduced within the internal guideline agreed upon with the materials testing institute of Nordrhein- Westfalen (MPA NRW). For the mechanical treatment for instance the parameters for roughness, flatness, and parallelism are reduced to half the values of the standards. This requires extremely sophisticated production, starting with the selection and production of suitable input stock, and including complicated heat treatment processes, extremely careful machining and calibration at the MPA NRW.
The hardness reference blocks used for indirect verification should conform largely to the work piece to be tested, in terms of material characteristics and hardness range.For this reason a hardness reference block made of aluminium was developed for the lower hardness range which can not be covered by steel, using new materials technology methods.
The division of MPA NRW responsible for calibration of hardness reference blocks has been accreditated as DKDcalibration Laboratory of Deutscher Kalibrierdienst (DKD) by the Physikalisch Technische Bundesanstalt (PTB), signatory of the multilateral agreement of the European Cooperation for Accreditation (EA) for mutual recognition of calibration certifcates. That ist one of the reasons why the calibrated hardness reference blocks distributed by Buderus are accepted without reservation not only in Germany but as well internationally.
When using hardness reference blocks it is irrelevant whether the value of the nominal hardness to be delivered corresponds exactly to the actual calibration value observed, since scale adaptation should be carried out with at least two hardness values.
A hardness reference block shall only be used as according to the standards to that method and test condition for which it was calibrated.
Hardness Reference Blocks made of Steel for the Rockwell Test
The method most commonly used for hardness testing of metal materials is the Rockwell hardness test,because of it's simplicity of operation and evaluation,and capability for automation. The hardness value is determined by measuring the permanent depth of indentation. The various hardness scales differ in terms of
> preliminary test force
> total test force
> calculation of the hardness value.
Depending on the particular test condition, the indenter used is a diamond cone or a hard metal alloy ball (W) or a steel ball (S).
Buderus Rockwell test blocks with a test certificate from MPA NRW are available for all standard test methods.
According to the new Rockwell standard the use of a square block with a larger thickness however is highly recommended.
Hardness Reference Blocks made of Steel for the Vickers Test
Vickers hardness test is comparable to that of Brinell, but the indenter used is a diamond pyramid with a square base. The hardness value is calculated using the test force and the length of the centre diagonal of the resultant impression.
Depending on the test force, a distinction is made in the Vickers test plates available between
> Micro blocks (HV 0.005 to HV 10)
> Macro blocks (HV 1 to HV 150)
Hardness Reference Blocks made of Steel for the Knoop Test
The Knoop hardness test is similar to that of Vickers. The indenter used is a diamond pyramid with rhombic base.
Hardness Reference Blocks made of Steel for the Brinell Test
In the Brinell test an indenter (ball) is pressed into the surface of a sample, and the resultant diameter of the indentation is measured. Since 1999 only hardmetal alloy balls (HBW) are used as indenters.
Buderus Brinell test blocks are produced in different sizes and different surface finishes depending on the ball diameter.
Hardness Reference Blocks made of Aluminium
For several years there has been a need for "soft"blocks. Using new materials technology methods, it is now possible to produce hardness reference blocks made of aluminium. They are available in the nominal hardness values 60 HV30,80 HV30 and 100 HV 30 in two different block sizes depending on the test.
Hardness Reference Blocks made of Steel for the Martens Hardness Test
The hardness testing method Martens HM (till now Universal hardness HU) is a registrating method,which incorporates the possibility for the determination of further materials parameters according to existing standards. The test is carried out force or indentation depth controlled with an indenter, which is in most cases a diamond according to Vickers. The method is especially used for thin hard layers and coatings.
Quality Insurances for Hardness Testing
For the quality insurance the standards published since 1999 recommend, as addition to annual calibration and verification, a daily check of the hardness testing machines. For documentation, calculation and presentation of the quality insurance for all hardness reference blocks distributed by Buderus an EXCEL-program has been created, in which e.g. the calibration values, the allowed values for the error and the repeatability of the hardness testing machine and the uncertainty of the reference block are integrated. This program and further information may be overtaken by download from URL: http://www.mpanrw.de/crm.When inserting the MPA NRW number and the hardness value of the reference block, the download can be started by a click on OK. When inserting "demo"on both positions a test version of one block as an example is downloaded. When using a hardness reference block together with the program for hardness testing machines the uncertainty of measurement and - according to the standard - the repeatability and the error are determined (Bibliographical References p. 8). In the program is controlled and displayed as text whether the values of the machine are according to the standard or not.
For the determination of uncertainty the program offers two possibilities
- one without the correction of the error according to UNCERT proposal of SMT of EU and
- one including the correction of error from nominal value.
Standards and Bibliographical References for Hardness Testing of Metal Materials
Buderus hardness reference blocks are supplied with an official calibration certificate (DKD) of the materials testing institute of Nordrhein- Westfalen (MPA NRW).The distribution to the end consumers is done only by the producers and distributors of hardness testing machines and service centres.
The hardness determined on calibration, the hardness scale, a registration number and the month and year of calibration are engraved on each block.
The test surface of each block is supplied with a "B" as the manufacturers'mark and additionally with a monitoring indentation marked with a circle for Vickers,Knoop and Brinell blocks.On request a screen on Brinell blocks (square/rectangular) and Rockwell blocks (square) can also be applied..
The employees responsible at Buderus Edelstahl and at the materials testing institute of Nordrhein-Westfalen will be pleased to respond to any enquiries and provide additional information at any time.
Special blocks, blocks with mill certificate and uncalibrated blocks on request.